Chapter 25 - Arenes Flashcards
Molecular formula of benzene?
C6H6
Any compound containing a benzene ring is a ?
Aromatic compound/ arene
What shape is the benzene ring?
Hexagonal
Where are the delocalised electrons in benzene?
In the Pi bond , moving above and below the hexagonal ring,
How are the delocalised electrons shown in benzene?
Circle within hexagon
What is the C-C-H bond angle in benzene?
120
What is the shape of a benzene molecule?
Planar - trigonal planar
Benzene is an unsaturated molecule this means
It doesn’t only contain single bonds
The benzene ring is named as phenyl when it’s?
Branched
differences between the electrophilic addition and electrophilic substitution?
EA vs ES:
milder reagents vs catalyst
more reactive vs less reactive
easier to substitute the Br2 across the c=c in EA but harder to substitute in benzene
Why is electrophilic addition more reactive and ES?
- Pi e-s are localised between the C=C -> induces a dipole in EA,
- but in ES, Pi electrons are delocalised over the ring - doesn’t have a high enough electron density to induce a dipole in the Br2.
% yield equation?
actual MOLES / theoretical moles x 100
Actual moles equation?
% yield x TM / 100
Tm equation?
actual moles x 100 / % yield
benzene only reacts w bromine if a ? is present.
catalyst. Alkenes don’t need a catalyst to be able to react w bromine tho
Since a catalyst is needed for benzene to react w bromine?
benzene doesn’t decolourise B water so it’s not a normal alkene
The C-C bond in benzene?
- all bonds are of equal length - 0.140
- the bond length is between that of a double and single bond
what is hydrogenation?
the addition of hydrogen
How much more stable is benzene than if it actually contained 3 ordinary C=C or structured lik previously expected?
It’s (360 -208) = 152 kJmol-1 more stable
* 360 comes from (3 x-120) which is the energy required to substitute 3H2s
how do we know double bonds don’t exist in benzene?
the hydrogenation of benzene can be measured experimentally and has been found to be -208 NOT -360 as would be expected if double bonds were present.
benzene structure?
- 6 Pi electrons delocalised along the ring, this stabilises the structure.
- all 6 p orbitals overlap SIDEWAYS
- Pi bonding
- one ring of 6 delocalised e-s
Evidence for benzene’s structure?
- C-C bond lengths are equal in the delocalised system
- The delocalised system reacts by substitution rather than addition
- hydrogenation energies are lower than expected bc benzene is more stable/less reactive than a ring structure w 3 C=C
- a benzene ring does not contain only single bonds so it’s unsaturated
Fridel Crafts reactions are?
just ES reactions. This reaction allows an alkyl group or an acyl group to be substituted onto a benzene ring.
Acyl group?
e. g. methanoyl C= Oxygen single bond H
e. g. ethanoyl CH3C=O
e. g. propanoyl CH3CH2C=O
⭐ IT’S REALLY IMPORTANT WHICH?
atom carries the + charge as this is the one that attaches to the benzene ring
if a compound contains a benzene ring?
it belongs to the homologous series arenes
what is an electrophile ?
a species w a + charge/ partial + charge that can accept a pair of e-s
in a substitution reaction?
a H is swapped on the benzene ring for another gr
Nitration of benzene forms?
nitrobenzene + H2O
Conditions for nitration of benzene?
- conc H2SO4 catalyst
- 50-60 degrees C - higher temp would result in further subs
reagents for nitration of benzene ?
conc HNO3
3 steps for ES?
1) form electrophile
2) curly arrows (2)
3) reforming catalyst
step 1 of nitration of benzene ?
HNO3 + H2SO4 -> NO2+ + H2O + HSO4-
what is the temp needed for the nitration of benzene?
room temp
how to work out what the electrophile is?
whatever gets added to the benzene ring
be careful of?
where the arrows start - must be accurate
halogenation of arenes conditions?
halogen carrier e.g. FeBr3
what are friedel- Crafts reactions?
- just ES
- allows an alkyl group/ acyl group to be subs onto benzene ring
how to form a acyl + electrophile?
- add Br to acyl group
- carry Br away w halogen carrier
alkylation/ acylation of a benzene ring condition?
halogen carrier
what is an acyl group?
group bonded to c=o
why do u getting multiple subs with Friedel Crafts Reactions ?
once 1 alkyl/ acyl gr attached to BR, it becomes activated meaning it will react by ES again so that more groups are added
bonding within phenol?
H
Phenol is a ? at room temp
solid
phenol solubility?
- not soluble in cold water
- soluble in hot water
- as temp inc, the higher energy used to break H bonds
H bonds strongest type of IMF so lots of energy required to break
solubility steps?
- break IMFS
- Form H bonds with molecules
‘like dissolves like’
pH of a warm solution of phenol?
5/6
- C6h5OH H2O on reversible arrow C6H5O- + H+
phenoxide ion formed
phenol is a ? acid?
- V weak acid
- reacts like typical weak acid almost - but can react like an acid or an arene
phenol reacting as an acid: with sodium
observations: gas bubbles/ fizzing, metals will dissolve and disappear
equation: 2C6H5OH + 2Na -> 2C6H5O-Na+ (sodium phenoxide) + H2
phenol reacting as an acid: with NaOH
C6H5OH + NaOH ➡ C6H5O-Na+ (sodium phenoxide) + H2O
weak acidic solution
add 1M of 0.5 NaOH to spatula of phenol
phenol reacting as an acid: conc HCl + sodium phenoxide
phenol reformed w NaCl as other product
phenol reacting as an acid: CuO
observations: black solid will get used up and blue solution will form
eq: 2 phenol + CuO -> H2O + (C6H5O)2Cu (copper (II) phenoxide)
phenol reacting as an acid: w NaCO3 (anhydrous)
no reaction
phenol is too weak an acid to release CO2 from a carbonate
ES of the BENZENE RING in phenol: Br2 (aq)
O: bromine water decolourised and WHITE PRECIPITATE FORMED
E: phenol + 3Br2 (aq) -> tribromophenol (?) + 3HBr
ES of the BENZENE RING in phenol: nitric acid?
dilute acid and no catalyst
O: orange precipitate forms on gentle heating
E: phenol + HNO3 -> 2-nitrophenol or 4-nitrophenol + H2O
ES of the BENZENE RING in phenol: nitric acid why
- 2-nitrophenol is the major product (get 2x as much of it)
- to get 2-nitophenol we can have NO2 on either 2 or 6 creates 2-
- but for 4- can only have NO2 on position 4
ES of the BENZENE RING in phenol: what type of reagents are involved?
dilute reagent + no catalyst
why is the benzene ring more reactive in phenol than in benzene?
- no halogen carrier needed like w benzene
- bc OH group attached directly yo the benzene ring
- this activates the ring causing phenol to be more reactive than benzene
reactivity of phenol?
- an electron pair (only one/2 of the lone pairs is at the right angle to b donated) from a p orbital on the O of the OH is donated to the benzene ring of phenol
- this increases the electron density in the ring and activates it
- phenol’s higher e- density allows it to induce a dipole in a halogen like Br2
phenol reacts more readily w electrophiles than benzene
Directing effect of OH group?
- OH group is activating (e- donating), has more effect on some positions than others
- OH group is 2,4 directing
- 2x as much sub in position 2 than 4
Directing effect of NH2 group?
- The NH2 group is electron donating & so actibvating
2,4 directing - 2x as much sub in position 2 than 4
Directing effect of NO2 group?
NO2 groups are electron withdrawing & have 3- directing effect
- it takes e-s out of the benzene ring, makes it harder to substitute onto the group
dissociation of phenol?
C6H5OH reversible arrow H+ + C6H5O-
phenol does not form many of these ion.
why does phenol not form many of the phenoxide ion?
- O is electronegative and so attracts the - charge (e-s)
- this attracts the H+ ions
- so more phenol molecules and less H+ formed
describe the structure of benzene?
- benzene has a trigonal planar shape w a bond angle of 120
- the C atoms are arranged in a hexagonal shape w the ring of delocalised e-s above and below the ring
describe the bonding in benzene?
- both sigma and pi bonds present
- sigma bond formed by the direct overlap of sp2 orbitals
- pi bond formed by sideways overlap of p orbitals
- forming a ring of 6 delocalised e-s above and below benzene ring
DRAW A DIAGRAM
Benzoic acid reacts like ?????? in nitration reaction?
benzene
⭐
positions r relevant to the group already attached (directing group)
